Verifying and Troubleshooting DHCP Configuration
Configuring a DHCP Helper Address
NOTE: The ip helper-address command will forward broadcast packets as a unicast to eight different UDP ports by default:
• TFTP (port 69)
• DNS (port 53)
• Time service (port 37)
• NetBIOS name server (port 137)
• NetBIOS datagram server (port 138)
• Boot Protocol (BOOTP) client and server datagrams (ports 67 and 68)
• TACACS service (port 49)
If you want to close some of these ports, use the no ip forward-protocol udp x command at the global configuration prompt, where x is the port number you want to close. The following command stops the forwarding of broadcasts to port 49:
Router(config)#no ip forward-protocol udp 49
If you want to open other UDP ports, use the ip forward-helper udp x command, where x is the port number you want to open:
Router(config)#ip forward-protocol udp 517
DHCP Client on a Cisco IOS Software Ethernet Interface
